Such a circuit configuration has come to knowledge from German Published Patent Application (DE-OS) No. 3234637. The handled signals of all wheel sensors are fed concurrently to two electronic circuits and are there processed by means of identical logic or rather in accordance with identical programs for the purpose of identifying malfunctions of the electronic circuitry. The signals available at different points in the course of the program are exchanged and checked for consistency. Any deviations are an indication of malfunctions wherefore in such a case either of the two electronic circuits signalizes this malfunction to one or several safety circuits. This causes a cut-off of the power supply to the solenoid valves serving for anti-lock control. As, in their rest positions, the solenoid valves do not influence the pressure medium supply to the brake and, hence, the brake application, nor permit any pressure removal via the outlet valves it is ensured that the vehicle will continue to be able to be braked, although without anti-lock control, in case of a trouble in the electronic system.
BRIEF DESCRIPTION OF THE INVENTION
However, in such a circuitry it is conceivable, although unlikely, that there are cases where anti-lock control will remain switched on despite a malfunction. Therefore, it is an object of the present invention to enhance the degree of safety even more, with which anti-lock control will be switched off in case of a trouble in the electronic or electric system, and thus to increase the safety of maintaining the braking operation, although without control. This object ought to be achieved without any additional expense or with very little additional expense at the maximum.
It has been found out that this object can be solved in a circuit configuration of the type referred to at the beginning in that, in case of consistency of the exchanged signals and in case of proper operation of the circuitry, the monitoring signal of each microcontroller is a predetermined alternating signal, i.e., an alternating signal with predetermined frequency and with predetermined variation; and in that the safety circuit compares the monitoring signal, or rather the alternating signal, with a time standard derived from one or several clock generators which are independent of the operating cycle of the microcontrollers.
According to this invention, the enhanced safety will thus be achieved in that an alternating is selected as monitoring signal by means of which each microcontroller signalized the proper condition to the safety circuit; and in that this alternating signal is compared with a time standard. The alternating signal, for instance, is a pulse sequence of predetermined duration and frequency. Moreover, a time standard, or rather a time window, is used for the monitoring signal of each microcontroller. Said time standard, or rather time window, is gained independently of the operating cycle of the microcontrollers by means of additional clock generators. These clock generators may be of simple construction as they only have to check relatively roughly whether the monitoring signal falls into the predetermined time window. Such clock generators, for instance, permit integration at low expense into the trigger circuit provided for the wheel sensor signals.
Should there be a failure of the clock generators defining the time windows this would also lead to a cut-off of anti-lock control. Thus the monitoring assemblies are also included in the monitoring operation.
According to one advantageous embodiment of this invention, the monitoring signal of each microcontroller is compared with the time window derived from a specific clock generator. As compared with the use of a common clock generator this will once more enhance the safety degree of error detection.
It will be an advantage if, upon error detection, the safety circuit interrupts the power supply path of a relay via the operating contact of which power supply to the solenoid valves takes place. A very expedient embodiment of such a circuit configuration for actuating the power supply relay is described in the German Patent Application No. P 39 24 988.3. If such a circuit configuration is used it will be ensured that power supply will be interrupted even in case of troubles of various types within the relay actuation system.
A further very advantageous embodiment of this invention consists in that, upon error detection, the safety circuit actuates semiconductor stages such as transistor stages via an additional signal output which block the actuation of the solenoid valves, e.g., by interrupting power supply to the driver stages, or rather amplifier stages, connected upstream of the solenoid valves. In this way, any further anti-lock control will be prevented even if the detected trouble admittedly will cause the safety circuit to respond, with the supply voltage, however, not being switched off because of a bridged switching contact, for instance.
According to a further embodiment of this invention, the microcontrollers will not only signalize defective operation if the mutually exchanged signals are not consistent but also if there appear signals or signal combinations which will not be possible in case of proper operation of the anti-lock control system. Monitoring thus also takes place in accordance with so-called plausibility criteria.
According to another favorable example of an embodiment of this invention, the output signal of a monitoring circuit is feedable to the microcontrollers, the error detection state of said output signal--e.g., a permanent signal instead of an alternating signal--being signalizable as malfunction to the safety circuit by means of the microcontrollers and of the monitoring signals. The output signal, for instance, may be an output signal of a parity chain monitoring the operation of the solenoid valves. A very advanced monitoring circuit of this type is described in German Patent Application No. P39 25 418.8.
Further characteristics, advantages and applications of this invention will become evident from the following description of one example of an embodiment, reference being made to the accompanying drawing.